The present invention relates to spectroscopy and more particularly a device for electrothermal atomization of a sample for spectroscopic analysis.
Electrothermal atomizers, commonly referred to as heated graphite atomizers or graphite furnaces, are utilized in atomic absorption spectrophotometers for rendering the sample to be analyzed in atomic form. Typically, the furnaces comprise a tubular graphite member clamped between annular graphite contacts or electrodes engaging its respective ends. A lateral opening in the side wall of the graphite tube serves as a sample port accommodating the insertion of the substance to be analyzed into the tubular member.
The graphite contacts, usually mounted in cooling jackets, are pressed into tight engagement with the ends of the tubular furnace member by resilient biasing means or a servomotor. An intense electrical current, passed longitudinally through the tubular member between the contacts, heats the member to the high temperature required to convert the sample to a "cloud of atoms". The sample is thereby atomized such that an atomic cloud is generated within the graphite tube.
A measuring light beam of a line emitting light source which comprises the resonant spectral line of a looked-for element is passed through the annular graphite contacts and the longitudinal bore of the graphite tube. The amount of the looked-for element in the sample can be determined from the absorption of the measuring light beam.
In order to prevent rapid deterioration of the tubular graphite member by oxidation at the high temperatures required for atomization of the analyte, provision is made for enveloping it in a flow of inert protective gas. The graphite tube is surrounded by an inert gas such that oxygen does not come into contact with the graphite tube.
A graphite furnace of the type just described is shown in Braun et al., U.S. Pat. No. 4,022,530 which is incorporated herein by reference. In this particular furnace, the contacts are tubular. The two contacts extend around the graphite tube along its entire length between the contact surfaces except for a separating gap. An inert gas flow is passed into the graphite tube from both ends. This inert gas flow emerges through a radial bore of the graphite tube in its center. One of the tubular contacts has a radial bore which is aligned with the radial bore of the graphite tube.
The contacts of the contact arrangement form a cavity wherein the graphite tube is held. Inert gas is introduced into this cavity and flows around the graphite tube. Two inert gas flows are thereby generated, one of which flows through the interior of the graphite tube, usually from the ends of the graphite tube inward and then exhausting through the central lateral opening (i.e., sample inlet port) and the other which flows around the outside of the graphite tube. A portion of the latter inert gas flow emerges through the gap which is necessarily formed between the two electrodes which have to be electrically insulated from each other. Thus, the passage of oxygen through the gap into the cavity and to the graphite tube is prevented. This gap causes quite a high consumption of inert gas. Additionally, disturbances occur whereby thermal decomposition products of the sample are guided over areas of relatively low temperature.
In Hutsch et al., U.S. Pat. 4,726,678 which is incorporated herein by reference and the publication in "Analytical Chemistry" 58 (1986), 1973, a graphite furnace is described in which the tubular furnace body has a rectangular cross-section and contact projections extend transversely to the axis of the furnace body. The furnace body and contact projections are formed as one integral graphite element. Current for heating the furnace is supplied through contact projections tranverse to the longitudinal axis of the tubular furnace body.
It is an object of the present invention to provide a new and improved electrothermal atomization furnace assembly which is particularly advantageous in atomic absorption spectroscopy.
Another object of the invention is to provide an electrothermal atomization furnace assembly which reduces the consumption of inert gas.
A further object of the invention is to provide such a furnace assembly which achieves a well-defined inert gas flow for effective protection of the furnace body against exposure to atmospheric oxygen and which prevents precipitation of sample components at relatively cold areas of the furnace assembly.
A further object of the invention is to provide such a furnace assembly which permits selective relative adjustment of inert gas flow over and within the furnace body.
Other objects will be in part obvious and in part pointed out more in detail hereinafter.
Accordingly, it has been found that the foregoing and related objects are attained in an electrothermal atomization furnace assembly having an electrothermal furnace body with contact surfaces configured for mounting engagement between coacting electrical contacts, a bore adapted for passing a measuring light beam therethrough and a sample port for introducing sample into the bore and exhausting inert gas flow and sample components therefrom. A pair of current-supplying contacts operationally mount the furnace body therebetween and are configured to form a cavity containing said furnace body and have a through-bore in alignment with the sample port. The contacts are in non-conductive spaced disposition so as to form a predetermined gap therebetween. Inert gas passageways in the contacts provide inert gas flow into the cavity for defined flow about and within the furnace body to prevent exposure to oxygen and remove sample components. An electrically insulating seal is mounted in said gap for sealing against inert gas loss from the cavity.
The furnace is arranged in a cavity which is closed . except for the though-bore aligned with the sample inlet port. From this closed cavity the inert gas flow can only emerge through the through-bore. Thus, the discharge of gas through an additional gap is prevented which results in a considerable economizing of inert gas and which also prevents flows which flow to such a gap. It has been found that such sealing of the gap between the contacts can be realized in practice despite the extremely high temperatures to which the furnace is heated in operation.
Advantageously the contacts are held in cooling jackets. Therefore, the contacts themselves, between which the seal is arranged, are relatively cold compared to the furnace itself. The seal is substantially shielded from the furnace by the cooled contacts. In this way, unacceptable heating of the seal by the radiation of the glowing furnace is counteracted.